can: mcp25xxfd: Add support for CAN reception

Add un-optimized CAN2.0 and CAN-FD reception support.

On a Rasperry pi 3 it is already able to process CAN2.0 Frames
with DLC=0 on a CAN bus with 1MHz without losing any packets
on the SPI side. Packets still get lost inside the network stack.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
[mani: misc cleanups for upstream]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>